1. Field of the Invention
The present invention relates to a resistive device whose electric resistance is adjustable as well as a method for its production.
2. Description of the Related Art
For numerous applications, resistive devices with a defined electric resistance must be produced in integrated circuits. There are several methods for the production of integrated resistive devices, which are, however, generally characterized either by a wide spread of the resistance values or by considerable processing requirements. In order to obtain a defined resistance value, resistive devices are often adjusted, or set, afterwards. Conventionally, however, this adjustment is only possible within a relatively small range.
An example of a conventional integrated resistive device is a film resistor. An insulating layer, for example a semiconductor oxide layer, is produced on a substrate. A thin semiconductor layer is deposited thereon and laterally structured. The thin semiconductor layer, or semiconductor film, is doped. A typical lateral shape of a film resistor is a rectangle. Along two parallel sides of the rectangle opposing each other, highly doped, for example p+-doped, contact strips are provided, via which the resistive device is contacted. The rectangular region of the semiconductor film located between these highly doped contact strips is the actual resistive region. The resistive region is doped less than the highly doped contact strips, but it has the same line type. Therefore, if the contact strips are p+-doped, the resistive region is preferably p−-doped.
The doping of both the resistive region and the contact strips is generally performed by implantation of a dopant with subsequent thermal activation of the implanted dopant.
In a film resistor of the described design, there are several possibilities, with a given geometry of the resistive device and with a given number of implanted dopant atoms, to change the electric resistance. One possibility is to change, or to vary, the dopant activation. For that, the temperature profile of the thermal dopant activation is varied, particularly the duration and the achieved temperatures. An incomplete activation of the dopant causes an increased resistance of the resistive device, as, on the one hand, only part of the dopant atoms is in lattice positions and, there, acts as donators or acceptors, respectively, and, on the other hand, dopant atoms in intermediate lattice positions and other lattice defects of the semiconductor produced during implantation of the dopant and not annealed act as traps for charge carriers and thus reduce the charge carrier density.
Another possibility is to cause, by thermal treatment, an out-diffusion of dopant from the highly doped contact strips into adjacent regions of the low-doped resistive region. Thereby the conductivity of the highly doped contact strips decreases only slightly, while the conductivity of the low-doped resistive region, at least near the highly doped contact strips, increases noticeably. The electric resistance of the resistive device is thereby reduced. Such an out-diffusion of dopant from the highly doped contact strips into the low-doped resistive region may be performed as early as in the process step of the dopant activation. By increasing the temperature or extending the duration of the dopant activation, an (increased) out-diffusion of dopant from the highly doped contact strips into the low-doped resistive region and hence a reduction of the electric resistance of the resistive device may thus be caused.
Another possibility is to produce, within the resistive device and particularly within the resistive region, first a dopant concentration that is non-homogeneous in a vertical direction, i.e. in a direction perpendicular to the semiconductor film, during the implantation of the dopant. Again by means of a subsequent thermal step, for example simultaneously with a dopant activation with changed parameters, a homogenization of the dopant concentration in vertical direction may be effected later. This, too, results in a change of the electric resistance of the resistive region, particularly in a reduction of the electric resistance.
A serious common disadvantage of the conventional ways described above for changing the electric resistance of a resistive device is that each thermal treatment has an effect on the whole substrate, or the whole semiconductor wafer, and devices possibly already formed or still to form therein. Therefore, temperature and duration of a thermal treatment cannot be varied arbitrarily, but must be kept within narrow limits. Accordingly, the electric resistance of a resistive device can only be changed within a very limited range in the ways described above. Furthermore, the described ways for changing the electric resistance of a resistive device are generally only applicable for resistive devices on the basis of semiconductor films on insulating layers which effectively prevent vertical diffusion. In the case of a resistive device formed in the substrate, the dopant would diffuse in a vertical direction and thus leave the space region provided for the resistive device.
Conventionally, a change of the electric resistance of an integrated semiconductor resistive device is therefore only possible within a limited resistance value range. In the case of technologies in which the electric resistance of the resistive devices varies more from wafer to wafer or from lot to lot, adjusting, or setting, the electric resistance to a desired value is therefore not possible.